Monopulse, also known as simultaneous lobe comparison, is a technique for determining the direction of arrival (DOA) of radiation. The radiation may emanate from an active source, for example, a transceiver, a transponder or a beacon, or a passive source, that is, a target or scatterer that reradiates some of the power incident on it. The working principle of the monopulse technique for DOA determination is described below with reference to FIG. 1.
Referring now to FIG. 1, a schematic top plan view of a conventional monopulse system 10 is shown. The monopulse system 10 includes a beacon 12 and a receiver 14. The beacon 12 is at a distance R from the receiver 14 and an angle θ from a boresight axis z of the receiver 14. The beacon 12 at position (R, θ) is received by identical first and second antenna patterns 16 and 18 at the receiver 14. The first and second antenna patterns 16 and 18 shown in FIG. 1 are realised by sequential lobing. However, although only one (1) receiver 14 is shown in FIG. 1, the first and second antenna patterns 16 and 18 can alternatively be realised by real simultaneous beams with two (2) receivers. The DOA θ of a signal from the beacon 12 is estimated based on a comparison of the amplitudes of the first and second antenna patterns 16 and 18, and more particularly by computing an angle estimator Δ/Σ denoting a ratio of the difference to the sum of the amplitudes of the first and second antenna patterns 16 and 18 with the following equation:
                              Δ          ∑                =                                                            f                1                            ⁡                              (                θ                )                                      -                                          f                2                            ⁡                              (                θ                )                                                                                        f                1                            ⁡                              (                θ                )                                      +                                          f                2                            ⁡                              (                θ                )                                                                        (        1        )            where f1(θ) represents the first antenna pattern 16, and f2(θ) represents the second antenna pattern 18. Monopulse technology is discussed in greater detail in “Monopulse Principles and Techniques,” by Samuel M. Sherman, Artech House, Inc. (1984) [1] and “Monopulse Radar,” by A. I. Leonov and K. I. Fomichev, Artech House, Inc. (1986) [2].
Monopulse technology is widely employed in radar applications, particularly in long-range radar applications having line of sight (LOS) conditions, for determining the angular location of a target. Most recently however, with the United States Federal Communications Commission's (FCC) approval of the commercial use of Ultra Wideband (UWB), an UWB-based monopulse system for position determination in an indoor environment has been proposed in Singapore Patent Application No. SG 200504866-5 by Sun Xiao Bing, et al. [3].
SG 200504866-5 [3] describes a UWB positioning system comprising a UWB transceiver and a UWB reference device. The reference device generates UWB response pulses in response to UWB pulses received from the transceiver. The transceiver includes multiple antennas and circuitry for determining the amplitudes of respective signals received by the antennas from the reference device in response to UWB pulse(s) transmitted from the transceiver. The signal amplitudes from the different antennas are compared to determine the DOA of the response pulse.
Referring now to FIG. 2, first and second waveforms 20 and 22 received by the prior art transceiver in an office environment with no large reflectors in proximity are shown. Samples of the first and second waveforms 20 and 22 (i.e. samples N1 to N2) are used in amplitude calculations for determining the DOA of the response pulse. Due to the short period of the UWB pulses (typically between about 1 to 2 nanoseconds), multipath effects are substantially suppressed by time-gating applied to the prior art UWB positioning system. Accordingly, the samples N1 to N2 are substantially from the direct path signal. However, in applications where the prior art UWB positioning system operates in proximity, for example, less than 1 meter (m), to a sizable reflector such as a wall, multipath signals resulting from mirror reflection of the response pulse off the reflector are typically of significant amplitudes and tend to interfere with the direct signal. Referring now to FIG. 3, first and second waveforms 24 and 26 received when the prior art transceiver is operating in proximity to a wall in the office environment are shown. In particular, the first and second waveforms 24 and 26 are collected at two antennas of the prior art transceiver, the two antennas having a 60 degree (°) squint angle therebetween, when the prior art transceiver is operating at a distance of 40 centimeters (cm) from the wall. Due to interference from multipath signals reflected off the wall, the first and second waveforms 24 and 26 in FIG. 3 differ substantially from the first and second waveforms 20 and 22 received by the prior art transceiver in the LOS application shown in FIG. 2. The presence of multipath signals introduces errors in the DOA of the response pulse estimated with samples N1 to N2 from the first and second waveforms 24 and 26 in FIG. 3.
In view of the foregoing, it would be desirable to have a method that provides a substantially accurate estimate of a DOA of a signal in applications with severe multipath.